Prepare the Cake Batter: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans to prevent sticking. In a large mixing bowl, whisk together 2 1/2 cups all-purpose flour, 2 1/2 teaspoons baking powder, and 1/2 teaspoon salt. In another large bowl, cream together 3/4 cup unsalted butter, softened and 1 3/4 cups granulated sugar until the mixture is light and fluffy, about 4-5 minutes. Add in 4 large eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon vanilla extract. Gradually mix in the dry ingredients combined with 1 cup whole milk, starting and ending with the dry ingredients. Lastly, fold in the 1 1/2 cups fresh raspberries that have been lightly tossed in flour.
Bake the Cakes: Evenly divide the cake batter into the prepared pans. Bake in the preheated oven for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Once baked, allow the cakes to cool in the pans for about 10 minutes before transferring them onto a wire rack to cool completely.
Make the Lemon Buttercream: In a large mixing bowl, beat 1 cup unsalted butter until creamy. Gradually add in 4 cups powdered sugar, beating until well combined. Mix in the zest and juice of 1 lemon, 1 teaspoon vanilla extract, and 2 tablespoons heavy cream. Beat until the buttercream is light, fluffy, and smooth, about 5 minutes.
Assemble and Decorate: Place one layer of the cooled cake on a serving plate. Use a spatula to spread a layer of the lemon buttercream over the top. Add the second layer of cake and frost the entire cake with the remaining buttercream. For decoration, scatter 1 cup fresh raspberries on top and arrange lemon slices on the sides if desired. Optional: Add buttercream swirls for an extra decorative touch.